The present invention relates to aromatic heterocyclic copolymers excellent in mechanical characteristics and suitable for use as structural materials for aircraft, automotive vehicles and space equipment, methods for producing the same, and methods for producing formed articles of the same.
The present invention further relates to molecular composite materials composed of matrix polymers and aromatic heterocyclic copolymers, methods for producing the same, and methods for producing formed articles of the same, and more particularly to molecular composite materials excellent in mechanical characteristics and suitable for use as structural materials for aircraft, automotive vehicles and space equipment.
In recent years, not only light alloys such as alloys of Ti, Al, etc., but also composite materials such as fiber reinforced plastics (FRPs) have been widely used in automotive vehicles and various vehicles, as well as aircraft, for the purpose of lightening weight. These composite materials have also been used as various kinds of structural materials, and composite materials excellent in mechanical characteristics have also been widely researched and developed.
In order to obtain composite materials excellent in mechanical characteristics, fiber high in strength and elasticity has been widely used as reinforcing materials to be added to matrices, and such fiber includes glass fiber and carbon fiber. Further, aromatic polyamide fiber termed so-called aramid fiber such as Kevlar (E. I. du Pont) have also been used.
Carbon fiber shows extremely high elasticity, but is inferior to aramid fiber such as Kevlar in loop strength and knot strength. On the other hand, aramid fiber has high strength, but is inferior to carbon fiber in elasticity. Then, new fiber having both the high elasticity of carbon fiber and the high strength and easy handling of aramid fiber has been desired, and research and development of such fiber have been made.
As substances meeting such requirements, heterocycle-containing aromatic polymers such as poly-p-phenylenebenzobisthiazole (polybenzothiazole; PBZ-T) and poly-p-phenylenebenzobisoxazole (polybenzoxazole; PBZ-O) are noted. Heterocycles are introduced into these substances as means for solving the problem of low elasticity, and hygroscopicity which is the disadvantage of Kevlar is also overcome thereby. Main chains of these substances exhibit rigidity, and can provide fiber having higher strength and elasticity.
Such polymers having heterocycles such as thiazole imidazole, oxazole and oxazinone rings in recurring units (hereinafter referred to as "polythiazoles, etc."), particularly polythiazoles containing thiazole rings, are high in rigidity, and have high strength, high elasticity and high thermal stability. They are therefore considered to be hopeful as plastic materials used as single bodies instead of metal materials, or as reinforcing materials for other engineering plastics.
However, rigid polymers such as polythiazoles, etc. are generally poor in solubility because of their high rigidity, and only dissolved in limited strong acids such as methanesulfonic acid and chlorosulfonic acid. Further, they are low in elongation, poor in bending properties, and thermally infusible, resulting in the difficulty of melt molding. It is therefore difficult to use them as single bodies.
Accordingly, the development of aromatic heterocyclic polymers has been desired which are improved in elongation and bending properties, soluble in organic solvent under mild conditions without use of strong acids, and miscible with other polymers, resulting in the possibility of general melt molding.
On the other hand, when other engineering plastics are compounded with reinforcing fiber, the strength of the resulting composite materials is known to largely depend on the interfacial adhesion between the matrix resins and the fiber, as well as the strength of the matrix resins and the fiber used as the reinforcing material. Under this situation, use of the matrix resins or the fiber having high strength and elasticity as materials does not necessarily result in acquisition of composite materials excellent in strength.
Then, attempts have been made to obtain so-called polymer blend composite materials (molecular composite materials) by finely dispersing rigid polymers such as aromatic polyamides as reinforcing polymers in polymers acting as matrix resins at a molecular level, thereby solving the above-mentioned problems.
However, even if the polymers having the above-mentioned heterocycles in recurring units are simply mixed with the matrix polymers to attempt to produce the molecular composite materials, sufficient miscibility of the reinforcing polymers with the matrix polymers can not generally obtained because of rigidity of the reinforcing polymers, resulting in difficulty of achieving homogeneous dispersion of the reinforcing polymers in the matrix polymers. If the reinforcing polymers are not homogeneously dispersed in the matrix polymers, molecular composite materials excellent in mechanical characteristics can not be obtained. Accordingly, various attempts have hitherto been made as described below.
For example, Japanese Unexamined Patent Publication No. 1-287167 discloses a method for producing a polymer composite comprising introducing a polymer solution mainly containing a reinforcing polymer (A) substantially consisting of a polythiazole having a rod-like skeleton and a matrix polymer (B) having the fusing property into a coagulation bath to form a film, wherein said polymer solution exhibits optical anisotropy, and is coagulated by way of an apparent optical isotropic phase after immersion in the coagulation bath.
Further, Japanese Examined Patent Publication No. 2-7976 discloses a polymer composition containing a reinforcing polymer (A) substantially consisting of a polythiazole having a rod-like skeleton and a matrix polymer (B) composed of slightly crystallizable aromatic copolyamide having a glass transition temperature of 200.degree. C. or more, an incipient fluidization temperature of 500 C. or less and an apparent crystal size of 25 .ANG. or less, crystals having said crystal size being formed when kept between the glass transition temperature and the incipient fluidization temperature for any time within 5 hours, in an (A)/[(A)+(B)] ratio of 0.15 to 0.70 (based on weight).
However, in the method for producing the polymer composite shown in Japanese Unexamined Patent Publication No. 1-287167 and the production of a composite material using the polymer composition disclosed in Japanese Examined Patent Publication No. 2-7976, homogeneous dispersion of the reinforcing polymers in the matrix polymers can not be expected so much, and the strength of the resulting molecular composite materials is not significantly improved. This is considered to be caused by insufficient dispersion of the reinforcing polymers in the matrix polymers because of poor miscibility of the reinforcing polymers showing rigidity with the matrix polymers.
Then, a method has been proposed which comprises homogeneously mixing a precursor of a rigid aromatic polymer with a matrix polymer or a precursor thereof in an organic solvent, not mixing the rigid aromatic polymer with the matrix polymer, and removing the organic solvent, followed by heating to convert the precursor to the rigid aromatic polymer (Japanese Unexamined Patent Publication No. 64-1760 and Japanese Unexamined Patent Publication No. 64-1761).
However, the inventors' researches revealed that, for example, when a molecular composite material was attempted to be produced using a thermoplastic resin as the matrix resin for the purpose of molding the molecular composite material by hot pressing, and using an aromatic polythiazole precursor therewith, by the method shown in Japanese Unexamined Patent Publication No. 64-1760 or Japanese Unexamined Patent Publication No. 64-1761 described above, an aromatic polythiazole formed by the thiazole ring-closing reaction coagulates in the heat molding stage of a homogeneous mixture of the matrix polymer and the precursor, resulting in reduced mechanical characteristics of the molecular composite material.
Further, although the use of the precursor improves the miscibility, it raises the problem that long-term stirring in an organic solvent is required in order to obtain the homogeneous mixture of the matrix polymer and the precursor. Furthermore, the aromatic polythiazole is poor in interaction with the matrix polymer, for example, in hydrogen bonds, so that the resulting molecular composite material is low in elongation. This causes a defect when it is used as a structural material.